Track mounted trolley systems are employed in a wide variety of applications including automated sorting and delivery systems in which delivery robots are utilized to receive and sort items to a variety of locations. One such system employing a plurality of track mounted robots in a sorting and delivery system is described in commonly assigned U.S. patent application Ser. No. 10/142,348 for an Apparatus and Method for Mail Sorting, filed Feb. 27, 2003, the disclosure which is incorporated herein by reference for all purposes. Various means of driving such track mounted units are known, including chain drives, belt drives and friction type traction drives where a wheel or pulley is rotated against the track to drive the unit. In many cases, these drive systems suffer from a number of drawbacks including excessive noise, inability to precisely position the trolley for loading or unloading, and excessive component wear. These drawbacks tend to be compounded when the unit being driven is heavy, requiring more force to accelerate, decelerate and precisely position the unit. The invention described herein addresses these shortcomings.